This invention relates to novel inorganic crystalline structures and their method of preparation.
When solutions of certain metals such as those found in Groups IVA and IVB of the Periodic Table of the Elements are mixed with oxyanions of elements found in Groups VA and VIB of the Periodic Table of Elements, amorphous gels having limited ion exchange characteristics are precipitated. Certain of these gels have been converted into stoichiometric crystalline phases by refluxing in a solution of a strong acid. Among the crystalline phases prepared in this manner are .alpha.-zirconium phosphate (hereinafter designated .alpha.-ZrP); see U.S. Pat. No. 3,416,884; and .beta. and .gamma.-zirconium phosphates (hereinafter designated .beta.-ZrP and .gamma.-ZrP, respectively), see Journal of Inorganic Nuclear Chemistry, 1968, Vol. 30, pages 2249-2258, Pergamon Press.
Such compounds have limited catalytic uses while exhibiting some degree of zeolitic (absorption) characteristics and poor temperature stability. Thus, heating of such compounds, such as up to 100.degree. C., removes the water of hydration without destroying the interlayer structure of the compounds. However, additional heating to higher temperatures causes condensation to occur with attendant breakdown in the crystalline structure and the ultimate creation of amorphous substances generally lacking in both zeolitic and catalytic characteristics.
It has now been found that through preliminary displacement of a replaceable hydrogen ion in the inorganic crystalline structure by a metal cation, (hereinafter sometimes referred to as ion exchange), the aforementioned high temperature instability is eliminated. In this manner, various structural changes can be accomplished in the inorganic crystal lattice without ultimate destruction of the crystalline nature of the compound. More specifically, when crystalline compounds of the formula: EQU M(OH).sub.z (HQO.sub.4).sub.2-z/2.xH.sub.2 O (1)
wherein M is a metal ion, Q is an anion, z is any value from 0 to 2 and x is an integer of from 0 to 8, and preferably from 1-5, are reacted (neutralized) with a cation, the resulting compounds have been found to retain a definite crystalline structure even when subjected to temperatures in excess of that necessary to drive off the water of hydration.
Typical metal ions intended to be included in the above-identified formula (1) as M are metal ions comprising elements selected from Groups IVA and IVB of the Period Table of Elements are disclosed on pages 392 and 393 of the Handbook of Chemistry and Physics, 35th Edition. Specific examples of suitable metal ions include silicon, germanium, tin, lead, titanium, zirconium, cerium, thorium, and hafnium. With respect to the anions set forth as Q in the aforementioned formula (1), suitable elements include those found in Groups VA and VIB of the aforementioned periodic Table of Elements. Typical examples of such materials include phosphorous, arsenic, antimony, bismuth, chromium, molybdenum and tungsten.
Although it sometimes happens that (HQO.sub.4) groups are replaced by hydroxyl groups to form compounds as defined by equation (1), preferred starting materials are free of hydroxyl groups, i.e., those where z is 0, wherein the formula is EQU M(HQO.sub.4).sub.2.xH.sub.2 O (2)
wherein M,Q and x have the definitions set forth above.
Although it is not intended that this invention be limited to any specific theoretical concept, it appears that replacing at least some of the replaceable hydrogen ions of the crystal lattice of the compounds of equation (1) with metal cations produces compounds of the following formula: EQU M(OH).sub.z Y.sub.t.sup.m+ (QO.sub.4).sub.2-z/2.xH.sub.2 O (3)
wherein m+ is an integer corresponding to the charge of the cation Y, t is a number such that mt=2-z/2, M, Q, z and x have the aforementioned definitions and Y is selected from hydrogen, ammonium, cations such as those found in Groups IA, IIA, IIIA, IVA, IB, IIB, IIIB including the lanthanide and actinide series, IVB, VB, VIB, VIIB, and VIII of the aforementioned Periodic Table of Elements, and mixtures thereof. Where Y is a mixture of cations, the term Y.sub.t.sup.m+ will be understood to include Y.sub.a.sup.m+ W.sub.b.sup.n+ wherein m+ is the charge of cation Y and n+ is the charge of dissimilar cation W so that am+nb equals 2-z/2 in equation (3). When b equals 0, equation (3) is applicable.
Again, with respect to preferred starting materials free of hydroxyl groups, formula (3) would be as follows: EQU MY.sub.t'.sup.m+ (QO.sub.4).sub.2.xH.sub.2 O (4)
wherein t' is a number such that mt'=2 and M, Y, Q, x and m are defined as set forth above.
Typical of the aforementioned cations are such elements as lithium, beryllium, sodium, magnesium, potassium, calcium, rubidium, strontium, titanium, cesium, maganese, molybdenum, barium, scandium, copper, zinc, chromium, aluminum, iron, cobalt, nickel, silicon, vanadium, lanthanum and actinium as well as the lanthanum and actinium series. Preferred are metal cations selected from the group consisting of alkali metals and alkaline earth metals, and having atomic numbers of at least 3 but not more than 20.
The aforementioned displacement reaction of this invention can be accomplished either through dry ion exchange, aqueous ion exchange, or through liquid-solid exchange with liquids such as titanium or tin tetrachloride or with metal salts dissolved in organic liquids. When the process employs dry ion exchange, it is not encumbered by the limitations present with aqueous ion exchange. Thus, it has been found that both anhydrous and hydrated salts can react directly in the solid state with the crystalline compounds defined by equations (1) and (2).
Thereafter, subsequent high temperature treatment of the exchange crystalline structure represented by formulas (3) and (4) apparently only releases the water of hydration from the ion exchange compounds. The condensation reaction (during which water would normally be split out of the compound) is thereby eliminated. The resulting compounds have been found to have characteristics similar to that of a molecular sieve material. Thus, depending upon the particular interlayer spacing, various cations can either be included or excluded. For this reason, it has now been found possible, by using the novel crystalline structures of this invention, to readily separate such molecules as water, amines, alcohols and the like from hydrocarbon solvents.
In still another aspect of the invention, it has been found that, depending upon the extent to which the initial ion exchange is accomplished, and furthermore depending upon the intensity of the heat treatment of the resulting ion exchange compounds, a plurality of phases of crystalline compounds can be produced, each having its own interlayer structure and x-ray diffraction pattern. For example, by partial replacement with one cation, followed by heat treatment to alter the structure and subsequent replacement of the first cation with a second dissimilar cation, various structures having properties not directly attainable by initial ion exchange are produced. Still further, washing out of the cations following the heat treatment leads to additional forms of crystalline compounds.
Summarizing the process of the present invention, the starting material can be any of the compounds encompassed by formula (1), above, whether in their known crystalline forms or, as has been found possible with .alpha.-ZrP, in an amorphous gel form. The initial step involves an exchange of metal cations for the replaceable hydrogen ions present in these compounds. It will be readily apparent to one skilled in this art that the exchange can be employed to replace all of the hydrogen ions present or only a small percentage such as about 10 to 20 percent. If the neutralization reaction is accomplished in an aqueous solution, ion exchange may be less than complete, i.e., less than 100 percent of theoretical exchange is accomplished. As a result, heating then produces mixtures of phases that appear to represent 1/2 (1 hydrogen replaced per molecule) and full exchange (both hydrogens exchanged in some molecules). It has also been found that the most useful metal cations for aqueous ion exchange are those elements classified in Groups IA, IIA, IIB and VIIB of the Periodic Table of the Elements.
In another aspect, it will be appreciated that the extent of ion exchange (cation loading), can be varied from zero to full exchange by starting either with the acid form or alternatively with the cation form and washing with an acid. Thereafter, the exchanged crystalline compound is heat-treated, first to remove any water of hydration, and thereafter to form new crystalline phases. Once any of the new crystalline phases are obtained, all or part of the first metal cation can be replaced with a second metal cation. For example, Li.sup.+ and Na.sup.+ forms of the exchangers are particularly useful for secondary replacement. Thereafter, any of the heat treated crystalline phases can be used as catalysts or ion exchange compounds. Furthermore, by washing out the cations used to retain the crystalline structure during the heat treatment, structures having highly desirable absorption characteristics (similar to molecular sieves) can be obtained.
By way of specific embodiment, the remainder of this disclosure is primarily directed to procedures for producing and creating novel phases, through ion exchange and treating, of various compounds and particularly crystalline zirconium phosphates (ZrP), such as prepared in the aforementioned publications, and especially .alpha.-ZrP, as prepared in U.S. Pat. No. 3,416,884. It will be readily apparent to one skilled in this art that the zirconium phosphates are merely representative of the compounds that can be obtained with any of the other metal ions, anions and metal cations set forth in formulas (1) and (2). For instance, Example IV E illustrates the modification of hydrated titanium phosphate; Example IV B the stabilization of hydrated zirconium arsenate while Example III A illustrates the preparation of a hydrated zirconium arsenate; and Example IIIB a hydrated tin phosphate.